Engine brakes or retarders are used to assist and supplement wheel brakes in slowing heavy vehicles, such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today's heavy vehicles.
Problems with existing engine braking systems include high noise levels and a lack of smooth operation at some braking levels resulting from the use of less than all of the engine cylinders in a compression braking scheme. Also, existing systems are not readily adaptable to differing road and vehicle conditions. Still further, existing systems are complex and expensive.
Known engine compression brakes convert an internal combustion engine from a power generating unit into a power consuming air compressor.
U.S. Pat. No. 3,220,392 issued to Cummins on Nov. 30, 1965, discloses an engine braking system in which an exhaust valve located in a cylinder is opened when the piston in the cylinder nears the top dead center (TDC) position on the compression stroke. An actuator includes a master piston, driven by a cam and pushrod, which in turn drives a slave piston to open the exhaust valve during engine braking. The braking that can be accomplished by the Cummins device is limited because the timing and duration of the opening of the exhaust valve is dictated by the geometry of the cam which drives the master piston and hence these parameters cannot be independently controlled.
In conjunction with the increasingly widespread use of electronic controls in engine systems, braking systems have been developed which are electronically controlled by a central engine control unit which optimizes the performance of the braking system.
U.S. Pat. No. 5,012,778 issued to Pitzi on May 7, 1991, discloses an engine braking system which includes a solenoid actuated servo valve hydraulically linked to an exhaust valve actuator. Hydraulic pressure (on the order of 3000 psi) is supplied by a high pressure hydraulic pump which supplies a high pressure plenum. A pressure regulator disposed between the high pressure hydraulic pump and the high pressure plenum maintains operating hydraulic pressure below a desired limit.
The servo valve disclosed in Pitzi '778 includes a high pressure source duct leading from the high pressure plenum, an actuator duct leading from the servo valve to the exhaust valve actuator and a drain duct. The servo valve has two operating positions. In a first or closed position, the high pressure duct is blocked and the actuator duct is in fluid communication with the drain duct. In this first position, pressure in the exhaust valve actuator is relieved through the drain duct to place the exhaust valve actuator in a rest position out of contact with the exhaust valve. In a second or open position, the drain duct is blocked and the high pressure duct is in fluid communication with the exhaust valve actuator.
The exhaust valve actuator disclosed in the Pitzi '778 patent comprises a piston which, when subjected to sufficient hydraulic pressure, is driven into contact with a contact plate attached to an exhaust valve stem, thereby opening the exhaust valve. An electronic controller activates the solenoid of the servo valve. A group of switches are connected in series to the controller and the controller also receives inputs from a crankshaft position sensor and an engine speed sensor.
U.S. Pat. No. 5,255,650 issued to Faletti et al. on Oct. 26, 1993, and assigned to the assignee of the present application, discloses an electronic control system which is programmed to operate the intake valves, exhaust valves, and fuel injectors of an engine according to two predetermined logic patterns. According to a first logic pattern, the exhaust valves remain closed during each compression stroke. According to a second logic pattern, the exhaust valves are opened as the piston nears the TDC position during each compression stroke. The opening position, closing position, and the valve lift are all controlled independently of the position of the engine crankshaft.
U.S. Pat. No. 4,572,114 issued to Sickler on Feb. 25, 1986, discloses an electronically controlled engine braking system. A pushtube of the engine reciprocates a rocker arm and a master piston so that pressurized fluid is delivered and stored in a high pressure accumulator. For each engine cylinder, a three-way solenoid valve is operable by an electronic controller to selectively couple the accumulator to a slave bore having a slave piston disposed therein. The slave piston is responsive to the admittance of the pressurized fluid from the accumulator into the slave bore to move an exhaust valve crosshead and thereby open a pair of exhaust valves. The use of an electronic controller allows braking performance to be maximized independent of restraints resulting from mechanical limitations. Thus, the valve timing may be varied as a function of engine speed to optimize the retarding horsepower developed by the engine.